No throw on duplicate

package.json

@@ -24,10 +24,11 @@
     "node": ">= 6.x"
   },
   "scripts": {
-    "test": "npm run prettier:check && npm run lint && npm run check && npm run testonly",
+    "test": "npm run prettier:check && npm run lint && npm run check && npm run testonly && npm run test:double-dependency",
     "test:ci": "npm run prettier:check && npm run lint -- --no-cache && npm run check && npm run testonly:cover && npm run build",
-    "testonly": "mocha -r ts-node/register --full-trace src/**/__tests__/**/*-test.js",
+    "testonly": "mocha -r ts-node/register --full-trace src/**/__tests__/**/*-test.js src/**/__tests__/**/*-test.ts",
     "testonly:cover": "nyc npm run testonly",
+    "test:double-dependency": "npm run build > /dev/null && mocha -r ts-node/register --full-trace src/__tests__/double-dependency.test.ts; rm -rf ./dist1",
     "lint": "eslint --cache --report-unused-disable-directives src resources",
     "benchmark": "node --noconcurrent_sweeping --expose-gc --predictable ./resources/benchmark.js",
     "prettier": "prettier --ignore-path .gitignore --write --list-different \"**/*.{js,md,json,yml}\"",
@@ -54,6 +55,10 @@
     "@babel/polyfill": "7.4.4",
     "@babel/preset-env": "7.5.5",
     "@babel/register": "7.5.5",
+    "@types/chai": "^4.2.0",
+    "@types/mocha": "^5.2.7",
+    "@types/node": "^12.7.2",
+    "@types/shelljs": "^0.8.5",
     "babel-eslint": "10.0.2",
     "chai": "4.2.0",
     "eslint": "5.16.0",
@@ -62,6 +67,7 @@
     "mocha": "6.2.0",
     "nyc": "14.1.1",
     "prettier": "1.18.2",
+    "shelljs": "0.8.3",
     "ts-node": "^8.3.0",
     "typescript": "^3.5.3"
   }

src/__tests__/double-dependency.test.ts

@@ -0,0 +1,35 @@
+import * as shelljs from 'shelljs';
+import { join } from 'path';
+
+import { expect } from 'chai';
+import { describe, it } from 'mocha';
+
+describe('graphql-js module', () => {
+  it('allows interop between two versions of iteslf', () => {
+    const warn = console.warn;
+    const warnedWith = [];
+    console.warn = message => warnedWith.push(message);
+    const root = shelljs.pwd().stdout;
+
+    const g1Path = 'dist';
+    const g2Path = 'dist1';
+
+    shelljs.cp('-r', g1Path, g2Path);
+
+    const g1 = require(join(root, g1Path));
+    const g2 = require(join(root, g2Path));
+
+    const obj = new g1.GraphQLObjectType({
+      name: 'Dog',
+      fields: { id: { type: g1.GraphQLString } },
+    });
+
+    expect(g1.isObjectType(obj)).to.be.true;
+    expect(g2.isObjectType(obj)).to.be.true;
+    expect(warnedWith[0]).to.include(
+      'Using GraphQLObjectType "Dog" from another module or realm.',
+    );
+    expect(warnedWith[1]).to.be.undefined;
+    console.warn = warn;
+  }).timeout(5000);
+});

src/index.d.ts

@@ -414,3 +414,6 @@ export {
   IntrospectionTypeRef,
   IntrospectionUnionType,
 } from './utilities/index.d';
+
+import instanceOf from './jsutils/instanceOf';
+export { instanceOf };

src/index.js

@@ -417,6 +417,9 @@ export {
   findDeprecatedUsages,
 } from './utilities';
 
+import instanceOf from './jsutils/instanceOf';
+export { instanceOf };
+
 export type {
   IntrospectionOptions,
   IntrospectionQuery,

src/jsutils/__tests__/instanceOf-test.ts

@@ -0,0 +1,38 @@
+// @flow strict
+
+import { expect } from 'chai';
+import { describe, it } from 'mocha';
+
+import instanceOf from '../instanceOf';
+
+describe('instanceOf', () => {
+  it('fails with descriptive error message', () => {
+    let calledWith = [];
+    const warn = console.warn;
+    console.warn = message => calledWith.push(message);
+
+    function getFoo() {
+      class Foo {}
+      return Foo;
+    }
+
+    function getBar() {
+      return Bar;
+    }
+
+    const Foo1 = getFoo();
+    const Foo2 = getFoo();
+
+    class Bar {}
+    const bar = new Bar();
+
+    expect(calledWith[0]).to.be.undefined;
+    expect(instanceOf(new Foo1(), Foo2)).to.be.true;
+    expect(calledWith[0]).to.not.be.undefined;
+    expect(instanceOf(new Foo2(), Foo1)).to.be.true;
+    expect(calledWith[1]).to.be.undefined; // only warn once
+
+    expect(instanceOf(bar, Foo1)).to.be.false;
+    console.warn = warn;
+  });
+});

src/jsutils/instanceOf.js.flow

@@ -0,0 +1,10 @@
+// @flow strict
+
+/**
+ * A replacement for instanceof which includes an error warning when multi-realm
+ * constructors are detected.
+ */
+declare export default function instanceOf(
+  value: mixed,
+  constructor: mixed,
+): boolean %checks(value instanceof constructor);

src/jsutils/instanceOf.ts

@@ -0,0 +1,48 @@
+/**
+ * A replacement for instanceof which includes an error warning when multi-realm
+ * constructors are detected.
+ */
+
+// Keep the message around, but only show it once (per instance of graphql-js that encounters mismatched constructors)
+let warned = false;
+// See: https://expressjs.com/en/advanced/best-practice-performance.html#set-node_env-to-production
+// See: https://webpack.js.org/guides/production/
+export default process.env.NODE_ENV === 'production'
+  ? // eslint-disable-next-line no-shadow
+    function instanceOf<T>(
+      value: unknown,
+      constructor: new () => T,
+    ): value is T {
+      return (
+        value instanceof constructor ||
+        (value != null &&
+          value.constructor != null &&
+          constructor.name != null &&
+          value.constructor.name == constructor.name)
+      );
+    }
+  : // eslint-disable-next-line no-shadow
+    function instanceOf<T>(
+      value: unknown,
+      constructor: new () => T,
+    ): value is T {
+      if (value instanceof constructor) {
+        return true;
+      }
+      if (value) {
+        const valueClass = value.constructor;
+        const className = constructor.name;
+        if (className && valueClass && valueClass.name === className) {
+          if (!warned) {
+            console.warn(
+              `@apollo/graphql: Using ${className} "${value}" from another module or realm.
+This could produce confusing and spurious results, especially if your "graphql"/"@apollo/graphql" versions are inconsistent.
+You can use \`npm ls graphql\` and \`npm ls @apollo/graphql\` to ensure all versions are consistent.`, // TODO: determine what we mean by "consistent"
+            );
+            warned = true;
+          }
+          return true;
+        }
+      }
+      return false;
+    };